Recently, a form of content utilization using a mobile device is becoming common, in which various software data, such as image data, audio data, and game programs (hereinafter referred to as content), is distributed from a data distribution server via a network such as the Internet to a user device, for example, a PC, or a mobile device such as a mobile computer, a PDA, or a cellular phone, the data being stored in a storage medium of the device.
An information device such as a PC or a mobile terminal has an interface for receiving content from a network, and it also includes control means that is needed to play back content, and a RAM and a ROM that are used as memories for storing programs and data.
Various content such as music data, video data, or programs is stored in a storage medium in a storage medium of a user device, such as a hard disk or a flash memory, via, for example, the Internet. The content is retrieved from the storage medium according to a user's instruction to an information device that is used as a content-playback device, such as a PC or a mobile terminal, and is played back by a main unit of the information device or by a display or a speaker that is connected.
When content is distributed, that is, when content is downloaded from a content distribution server to a user device, usually, an authentication process of various types as to whether the user device is a legitimate device that is allowed to access content is executed, for example, based on a public-key cryptosystem or a shared-key cryptosystem, thereby verifying the authenticity of the user device.
In accordance with the recent spread of various information processing terminals, use of a plurality of information devices by an individual user has become common; for example, an individual user has a PC at home, and uses a mobile device outside such as a cellular phone, a mobile computer, or a PDA. It has become possible to issue requests for downloading various content from the various devices to content distribution servers.
When content is downloaded on condition of authentication, for example, when a user using a mobile terminal outside issues to a content distribution server a request for content distribution, authentication is executed between the mobile terminal the user is using and the server, and content is sent when the mobile terminal is a legitimate device. When the user using a PC at home issues to the content distribution server a request for content distribution, authentication is executed between the PC the user is using and the server, and content is sent and downloaded if the PC is a legitimate device. That is, the content distribution server permits downloading of content by executing authentication with a device that serves as a content-download destination.
Types of content to be downloaded have become diverse, and content available for downloading includes small-volume content that can be stored in a small memory of a cellular phone or the like, such as music content or a game program, and large-volume content that can be stored in a hard disk of a PC or the like but cannot be stored in a small memory of a cellular phone or the like, such as moving-picture content of a movie or the like.
If a user using a mobile terminal having a relatively small memory, such as a flash memory, issues a request to a content distribution server for downloading large-volume content such as a movie, the content cannot be stored in the memory of the mobile terminal, so that the user inevitably has to give up downloading the content.